zfs boot: try to set vfs.root.mountfrom from currdev as a fallback
zfs boot: try to set vfs.root.mountfrom from currdev as a fallback

This way with the new zfsloader there is no need to explicitly set zfs
root filesystem either via vfs.root.mountfrom or fstab.
It should be automatically picked up from currdev which is by default
is set from bootfs.

Tested by:	Florian Wagner <florian@wagner-flo.net> (x86)
MFC after:	1 month

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